My 11mm(mercury) rope is an end from a treestuff bag, and only 60' long. Long enough to give it a good trial climb, but I would have had to anchor it(finish the tie off) using the throwline, and leave a throwline tagline on it to remove when finished. It was going through a bunch of lower limbs, and it would have been a real hassle trying to get everything setup to work with a big loop of rope/cord. Dunno if you have pin oaks out that way, but the branches grab and stick to everything, so it's really hard manipulating cord through them, or even tossing cut limbs as far as that goes. The tree grabs them, and they hang up 10' below you :^D
As to the why 11mm... I've soley used ½" rope since I started. I like the hand feel, but I don't like the weight so much with long lengths. It's also hard for other people to use my rope with mechanicals on a rec climb, where you might want to swap lines for a different climbing experience. I can use anything since I use cord, but mechanicals have to be reset for the larger rope, and it's a PITA. I've been thinking about buying a 11mm kernmantle, but I want to give 11mm a good trial before I spend the money. I'm happy with my km3, and the weight isn't that big a deal if I drastically prefer the larger size, but it would be nice shaving a couple pounds, and getting a little more flexibility.